Java Virtual Machine (Java Series)

dup

dup Stack

This pops the top single-word value off the operand stack, and then pushes that value twice - i.e. it makes an extra copy of the top item on the stack.

This instruction cannot be used to duplicate two-word items (longs or doubles) - use dup2 instead.

Example

; This is like the java expression: ; ; StringBuffer x = new StringBuffer(); ; ; Make a new StringBuffer object and leave a reference to it ; on the stack: new java/lang/StringBuffer ; [ Stack now contains: objectref ] ; Duplicate the object reference: dup ; [ Stack now contains: objectref objectref ] ; Invoke the object's initializer: invokespecial java/lang/StringBuffer/<init>()V ; [ Stack now contains: objectref ] ; Store the objectref in local variable 1. astore_1 ; [ Stack is now empty. ] Bytecode

dup2, dup_x1, dup2_x1, dup2_x2

Java Virtual Machine, by Jon Meyer and Troy Downing, O'Reilly Associates